The present invention relates to position monitoring systems for vessels and, more particularly, to a radar monitoring system which alloys a vessel to determine its position in a radar image of in the monitored area.
Because small craft, viewed from any distance, have their natural reflecting sources very near the sea surface, their radar echoes are severely reduced by propagation phenomena. The "propagation factor" for such targets may be as much as 20 to 40 dB below unity, so that the echo is that far below the echo from a similar object in free space, at the same range from the radar transmitted/receiver.
As a result, radar reflectors are commonly used to enhance the echo of small craft. Often these take the form of a cluster of cube corner reflectors supported in an elevated position to provide good reflection of radar raves transmitted from other vessels. Elevation of the reflector to a position near the top of the mast of a small sailboat or on a rover or mast on a power boat can reduce most or all of this loss, giving a clear echo to approaching ships and to land based radar. Use of such reflectors is recommended as a safety precaution for small craft operating in sea lanes and other areas traversed by radar equipped ships.
Although navigational radar units for small craft have become smaller and less expensive over the past several years, it has also been proposed to use a large, shore based radar station which broadcasts a radar display on a television channel to a relatively low cost television monitor on the vessels in the area. This would provide a reduction in the size, cost, and complexity of the small craft equipment. Thus, only a simple television receiver would be required to obtain the navigation display, and contamination of the electromagnetic spectrum by multiple radar installations can be avoided.
A major limitation of such radar/TV systems is the inability of a vessel to identify the echo on the display which represents it. To overcome this limitation, Schrader U.S. Pat. No. 2,655,648 and Stocker U.S. Pat. No. 2,760,187 proposed that each craft carry an active transponder to produce a distinctive signal. This not only imposes an economic burden on the user, but also creates additional interference in the radar band which could become severe if many transponder equipped users were present in one area.
Radiation from auxiliary antennas as a means of marking the location of the user's own craft on the display has been proposed in Stavis U.S. Pat. No. 3,111,663, Gamertsfelder et al U.S. Pat. No. 3,111,664 and Roberts et al U.S. Pat. No. 3,121,223.
If the television scanning and transmission are synchronized with the radar scan, rather than having the TV system operating in its normal raster mode, the radar beam passing across the target may be detected by a supplementary receiver, and the output of that receiver used to brighten the display along the line pointing to the target echo, as shown in Smith U.S. Pat. No. 3,298,022, Cooper U.S. Pat. No. 3,404,397, Ennis et al U.S. Pat. No. 3,836,961 and Hendrickson U.S. Pat. No. 4,336,539. However, this requires a specialized TV transmission and receiving system, operating with a polar coordinate scan.
Assuming that a radar reflector provides an adequate echo for land based navigational radar monitoring the area, the navigator's problem remains that of identifying the vessel's own echo in a display filled with echoes from other craft. If the reflector were easily reached, it could be covered briefly and intermittently with an absorbing material to give sharp reductions in the echo signal. However, the reflector should be high on the mast, or as high as possible on a powered craft, and accordingly not readily accessible to the navigator. Electrical control is most desirable, so that the reflector can be modulated briefly when a pushbutton is depressed by the navigator. Then, having recognized the echo return from the vessel, the operator could return the reflector to its normal operation as a passive safety device, leaving the same identification capability available to other users without confusion.
Accordingly, it is an object of the present invention to provide a novel assembly for placement upon a vessel enabling determination of its position within a televised image of a radar scan of a monitored area.
It is also an object to provide such an assembly wherein the determination procedure is effective without additional transmission or reception of special signals by the using vessel or the base station.
Another object is to provide a vessel monitoring system utilizing a base station which is relatively low cost to equip and which may be operated automatically and at relatively low cost.